Statement/Schema builders should be extended with Scylla-specific options

[avelanarius]

The existing SchemaBuilder/statement builders facilites allow for convenient creation of Scylla statements. Unfortunately, many of Scylla-specific extensions are missing from those builders.

For example, it's currently not possible to use SchemaBuilder to create a CREATE TABLE statement with per_partition_rate_limit. Trying it:

Statement stmt = SchemaBuilder.createTable("ks.my_table").addPartitionKey("pk", DataType.cint()).withOptions().freeformOption("per_partition_rate_limit",
        "{'max_reads_per_second': 123, 'max_writes_per_second': 456}");

results in creating such a query:

CREATE TABLE [...] WITH per_partition_rate_limit = '{'max_reads_per_second': 123, 'max_writes_per_second': 456}'

And per_partition_rate_limit value dictionary is unnecessarily escaped with ' '.

We should add convenience methods for all Scylla-specific CQL extensions.

Refs #155